The development of pistons for a well known series of two cycle diesel engines manufactured for use in railway locomotives and other applications has extended over many years. Continuing modifications in the engine design, involving among other things higher power output per cylinder, have created increasingly severe operating requirements of pressure and temperature which have from time to time required modifications in piston design to maintain the record of extended durability desired. Considerable background information about the history and development of pistons for engines of this type is found in U.S. Pat. No. 3,240,193 issued Mar. 15, 1966, corresponding Canadian patent No. 771,421 issued Nov. 14, 1967 and Canadian patent No. 963,752 issued Mar. 4, 1975, all assigned to the assignees of the present invention.
A prior piston design, shown in the Canadian No. 963,752 patent provided an oil cooled floating piston construction having a thin walled crown including a recessed combustion bowl surrounded by a rim defining a squish land. The rim portion of the crown was connected with a cylindrical wall including an annular heat dam of limited cross section, a ring belt and a side thrust absorbing cylindrical skirt portion.
The prior piston design provided rigidity in the crown and ring belt structure through use of a plurality of thin radially extending nonperforate gussets connecting an internal thrust collar directly with the interior bowl and rim portions of the piston crown and the heat dam and ring belt portions of the skirt-defining cylindrical wall. Additional cooling fins provided between gussets in the rim and heat dam area joined with the gusset structure to carry heat from the connected surfaces of the piston rim and combustion bowl for transmission to cooling oil directed against the interior wall surfaces of the piston to maintain adequate cooling of the piston walls.
In conjunction with continuing engine improvements accompanied by a further increase in cylinder power output and a resultant increase in thermal loading on the pistons, it was determined that even greater cooling effectiveness should be provided in the piston crown at the location of the annular arcuately curved wall, or radius, at the inner edge of the crown rim which joins the surrounding planar squish land with the outer portions of the recessed combustion bowl. This "rim radius" portion of the piston is generally the hottest area in the crown of a direct injection diesel engine piston utilizing a symmetrical fuel spray pattern. This is partly because the fuel spray pattern and the combustion bowl configuration direct the major portion of combustion into the outer reaches of the combustion chamber. Then downward motion of the piston during the power stroke results in a high speed turbulent flow of extremely hot gas outward over the rim to fill the space between the piston squish land and the cylinder head as the piston moves away from the head which defines the cylinder closed end. Added to this is the relative difficulty of cooling a salient corner of the piston crown wall which has a much greater surface area exposed on the hot combustion chamber side than is exposed to coolant in the piston undercrown. In short, the rim radius lies in an area of high heat input to the piston and, due to the piston geometry, it is difficult to cool.
If the temperature of a piston becomes excessive during operation at maximum power settings of an engine, the interior surface may become hot enough to partially oxidize the cooling oil and create carbon deposits on the interior surfaces in the high temperature crown rim. The carbon layer built up thereby reduces the effectiveness of oil cooling and further raises the rim surface temperature. This may result in physical and metallurgical effects which eventually produce surface cracking in the area of the hot rim radius and may result in limiting the life of the piston.